Five winter wheat cultivars, differing in resistance to Septoria tritici blotch (STB), were spray inoculated under field conditions for two years and at two locations with nine Mycosphaerella graminicola isolates and a mixture of isolates that were obtained from different regions of the Czech Republic. Main aims of this study were (i) to compare isolate, host cultivar and environmental effects on five traits indicative of STB severity and (ii) to analyse pathogen aggressiveness and host-pathogen relations for improving evaluation of cultivar resistance. ANOVA showed in all traits, except the reduction in 1000 grain weight, significant isolate effects. However, the effects of isolate and genotype by isolate interactions were much lower (2.3–4%) than cultivar (19.1–53.7%) and environmental (11.9–58.6%) effects. Cultivar resistance to the disease limited much the loss in grain weight per spike to a halve, from 34.7% in the susceptible Bakfis to 17.3% in the resistant Arina. Visual scoring of symptoms in the middle and at the end of disease development (performed on the 1–9 scale), reflecting the disease progress and infected leaf area, showed the highest cultivar effect (54%) and could be recommended for evaluation of cultivar resistance in breeding practice. All examined traits were significantly interrelated, but significant differences between all the five cultivars were only detected after examination of the % coverage of flag leaves with lesions bearing pycnidia. Resistance in the cultivar Arina was detected by all isolates and the isolate mixture. In spite of significant differences in classification of resistances in the cultivars Bohemia and Mulan after inoculation with one isolate (1081), specific interactions between cultivars and isolates collected in this Central European region are rare. The study leads to a conclusion that investigation into stability of STB resistance across a wide range of environments is more valuable for breeding purposes than the study of cultivar response to different isolates. Combination of important isolate properties in a mixture of isolates is stressed as well.
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